**1. n. [Geophysics]**

In acoustics, the product of velocity times density, also called acoustic impedance and symbolized by *Z*. The reflection coefficient of an interface depends on the contrast in acoustic impedance of the rock on either side of the interface.

**2. n. [Geophysics]**

In electromagnetics or electrical circuit theory, the ratio of voltage to current when these are represented by phasor quantities in alternating current circuits. (A phasor is a complex number that represents the amplitude and phase of a quantity that varies sinusoidally in time.) Electrical impedance, also symbolized by *Z*, is a complex number that has the same units (ohms) as resistivity.